1. Field of the Invention
The present invention relates to a control apparatus for a hydraulically operated vehicular transmission having a plurality of transmission trains to be established by a selective operation of a plurality of hydraulic engaging elements.
2. Description of Related Art
As a control apparatus for this kind of transmission, there is conventionally known the following. Namely, a control apparatus is provided with a hydraulic control circuit for controlling the supply and discharge of hydraulic oil to and from a plurality of hydraulic engaging elements, and an electronic control circuit in which there are stored mapped speed change characteristics to be defined by parameters of an engine load such as a throttle opening degree, and a vehicle speed. By means of the electronic control circuit a transmission train corresponding to the throttle opening degree and the vehicle speed at the present moment is selected, and an order signal is outputted to the hydraulic control circuit. The hydraulic oil is then supplied to the hydraulic engaging element that was selected so as to establish the selected transmission train, thereby effecting an automatic speed changing.
The speed change characteristics define an upshift characteristic line (upshift line) and a downshift characteristic line (downshift line) between respective two transmission trains that are next to each other in the order of speed. In the hysteresis region between the upshift line and the downshift line it becomes possible to establish both transmission trains on the low-speed side and the high-speed side. Especially, there is recently a tendency of enlarging the region in which the high-speed transmission trains can be established in order to improve the specific fuel consumption. In this case, if the speed change characteristics as shown in FIG. 2 are employed, there will be a region in which three or more transmission trains can be established. It becomes therefore difficult to judge which transmission train should be established.
In order to solve this problem, there is known, as in Japanese Published Unexamined Patent Application No. 190960/1988, an art in which the following procedure is followed. Namely, the speed change characteristics and the throttle opening degree as well as the vehicle speed at the present moment are compared, and the lowest speed transmission train SL that can be established and the highest speed transmission train SH that can be established from the viewpoint of speed change characteristics are picked up. When the speed of the presently established transmission train SO is in the relationship SL.ltoreq.SO.ltoreq.SH, the transmission train SO is continued to be established, and when SO&lt;SL, upshifting is effected from SO to SL, and when SO&gt;SH, downshifting is effected from SO to SH.
In this prior art, when the running condition changes, while running at the fifth speed at point A in FIG. 2, sequentially to points B, C, D, E and F by increasing the depression of an accelerator pedal, the fifth speed will be maintained at point B because SL=the fourth speed, SH=the fifth speed and SL.ltoreq.SO.ltoreq.SH. The fifth speed will be maintained at point C because SL=the third speed, SH=the fifth speed and similarly SL.ltoreq.SO.ltoreq.SH. At point D, downshifting from the fifth speed to the third speed will be effected because SL=SH=the third speed and SO&gt;SH. At point E, the third speed will be maintained because SL=the second speed, SH=the third speed and SL.ltoreq.SO.ltoreq.SH. At point F, downshifting from the third speed to the second speed will be effected because SL=the first speed, SH=the second speed and SO&gt;SH. Then, when the running condition sequentially moves from point F to points E, D, C, B and A by releasing or decreasing the depression of the accelerator pedal, the second speed will be maintained at point E because SL.ltoreq.SO.ltoreq.SH. At point D, upshifting from the second speed to the third speed will be effected because SO&lt;SL. At point C, the third speed will be maintained because SL.ltoreq.SO.ltoreq.SH. At point B, upshifting from the third speed to the fourth speed will be effected because SO&lt;SL. At point A, upshifting to the fifth speed will be effected because SO&lt;SL=SH.
Normally, there will exist a time lag from the time at which the supply of the hydraulic oil is started to the time at which the hydraulic engaging elements come into substantial engagement. Here, in case the running condition transfers from point A to point D in FIG. 2, thereby starting the downshifting to the third speed, and thereafter immediately transfers to point B, upshifting to the fourth speed will be effected before the hydraulic engaging element for the third speed comes into substantial engagement. It follows that the amount of transmission of the driving force decreases until the hydraulic engaging element for the fourth speed has come into substantial engagement. At this time, the hydraulic engaging element for the fifth speed still has some hydraulic pressure left therein because only a short time has lapsed from the point of time of downshifting to the third speed. Therefore, if an upshifting is made to the fifth speed that can be established at point B, the hydraulic engaging element for the fifth speed will quickly be brought into engagement, with the result that the interval from the time of downshifting to the third speed to the time at which the driving force is sufficiently transmitted becomes short. In the above-described prior art, however, there will be effected only the upshifting to the fourth speed when the running condition has transferred from point D to point B. Therefore, a relatively long time is required until the driving force is sufficiently transmitted, resulting in a poor drivability.